Dont' require IF_MATCH header

The HTTP RFC doesn't require the IF_MATCH header and it's language on
what to do when it's not included isn't absolute so removing this check.

The implementing codebase can easily add this by overriding
precondition_check in ETagCacheLookup if necessary.

rest_framework/exceptions.py

@@ -96,14 +96,6 @@ class PreconditionFailed(APIException):
         self.detail = detail or self.default_detail
 
 
-class IfMatchMissing(APIException):
-    default_detail = 'IF_MATCH header is required'
-    status_code = status.HTTP_400_BAD_REQUEST
-
-    def __init__(self, detail=None):
-        self.detail = detail or self.default_detail
-
-
 class ConfigurationError(Exception):
     """
     Indicates an internal server error.

rest_framework/views.py

@@ -313,15 +313,6 @@ class APIView(View):
             headers.update(cache_lookup.get_response_header(obj))
         return headers
 
-    def check_update_validity(self, request):
-        """
-        """
-        # TODO add setting to cover
-        #  * raise IfMatchMissing when it's missing (if it's there, carry on)
-        #  * continue regardless
-        if request.META.get('HTTP_IF_MATCH') is None:
-            raise exceptions.IfMatchMissing
-
     def cache_precondition_check(self, obj, request):
         for cache_lookup in self.get_cache_lookups():
             cache_lookup.precondition_check(obj, request)
@@ -430,10 +421,6 @@ class APIView(View):
             else:
                 handler = self.http_method_not_allowed
 
-            if request.method.lower() in ('put', 'delete'):
-                # FIXME this method name isn't obvious
-                self.check_update_validity(request)
-
             response = handler(request, *args, **kwargs)
 
         except Exception as exc: