Check if model is a placeholder before we override existing CoreData properties
Revert "Check if model is a placeholder before we override existing CoreData properties"

This reverts commit 0c52681f3648d93ccb8419f70ba7427dfd7c6502.
Merge branch 'master' of github.com:MantleFramework/Mantle into hack/check_if_placeholder_before_overwriting_coredata
MTLManagedObjectSerializing now allows merging values before overwriting
them.
Check if we need to merge with an existing NSManagedObject a bit sooner
Remove guard clause for a case that doesn't really occur
Unfocus test
Small refactoring of test for legibility
Use constant string instead of NSStringFromSelector() in test
Move respondsToSelector check outside of loop

This way we only need to check once, instead of for each propertyKey.
Document mergeValueForKey:fromManagedObject and mergeValuesForKeysFromManagedObject:
Minor style fix
Merge pull request #272