Better handle conflicting keys in set_values

This commit adds logic to set_values to move values on keys to be added
upon to a blank key inside the value's previous key. This would allow
constructions like:

```
fk_object = HyperlinkedRelatedField(source='fk')
fk_name = CharField('fk.field')
```

Tests are also added specifically for set_values with the examples given
in the function description as well as for this change.

rest_framework/fields.py

@@ -125,9 +125,14 @@ def set_value(dictionary, keys, value):
     for key in keys[:-1]:
         if key not in dictionary:
             dictionary[key] = {}
+        elif type(dictionary[key]) is not dict:
+            dictionary[key] = {'': dictionary[key]}
         dictionary = dictionary[key]
 
-    dictionary[keys[-1]] = value
+    if keys[-1] in dictionary and type(dictionary[keys[-1]]) is dict:
+        dictionary[keys[-1]][''] = value
+    else:
+        dictionary[keys[-1]] = value
 
 
 def to_choices_dict(choices):

tests/test_fields.py

@@ -14,7 +14,7 @@ from django.utils.timezone import activate, deactivate, override, utc
 import rest_framework
 from rest_framework import exceptions, serializers
 from rest_framework.fields import (
-    BuiltinSignatureError, DjangoImageField, is_simple_callable
+    BuiltinSignatureError, DjangoImageField, is_simple_callable, set_value
 )
 
 # Tests for helper functions.
@@ -2380,3 +2380,65 @@ class TestValidationErrorCode:
                 ),
             ]
         }
+
+
+# Tests for set_value function
+# ----------------------------
+
+class TestSetValue:
+
+    def test_no_keys(self):
+        """
+        If no keys are provided, but a dict as value, add the dicts
+        """
+        d = {'a': 1}
+        set_value(d, [], {'b': 2})
+        assert d == {'a': 1, 'b': 2}
+
+    def test_one_key(self):
+        """
+        If a key + value provided, add the value to the dict with key
+        """
+        d = {'a': 1}
+        set_value(d, ['x'], 2)
+        assert d == {'a': 1, 'x': 2}
+
+    def test_many_keys(self):
+        """
+        With many keys, add the item to the in-most dict
+        """
+        d = {'a': 1}
+        set_value(d, ['x', 'y'], 2)
+        assert d == {'a': 1, 'x': {'y': 2}}
+
+    def test_many_keys_existing(self):
+        """
+        With many keys with existing in-built dict
+        """
+        d = {'a': 1, 'x': {'a': 2}}
+        set_value(d, ['x', 'y'], 3)
+        assert d == {'a': 1, 'x': {'a': 2, 'y': 3}}
+
+    def test_conflicting_keys(self):
+        """
+        If a value exists where a key will be added, use a blank key for old value
+        """
+        d = {'a': 1, 'x': 2}
+        set_value(d, ['x', 'y'], 3)
+        assert d == {'a': 1, 'x': {'': 2, 'y': 3}}
+
+    def test_reverse_conflict(self):
+        """
+        If a dict exists and a value is to be added, add it as blank key
+        """
+        d = {'a': 1, 'x': {'y': 2}}
+        set_value(d, ['x'], 3)
+        assert d == {'a': 1, 'x': {'y': 2, '': 3}}
+
+    def test_overwrite_conflict(self):
+        """
+        If a newer final value comes, replace with the older
+        """
+        d = {'a': 1, 'x': 2}
+        set_value(d, ['x'], 3)
+        assert d == {'a': 1, 'x': 3}