Fixed a bug where RemoveFieldsDefaultValuesRefactoring removes a non null initializer of a non primitive field.

RemoveFieldsDefaultValuesRefactoring.java:
Fixed the bug.
Renamed isConstant() to isConstantField().

RemoveFieldsDefaultValuesSample.java:
Added a non regression test.
Improvement: InvertEqualsRefactoring can invert equals when using an enum value.

InvertEqualsRefactoring.java
Added isConstantValue().

InvertEqualsSample.java:
Added a test.