Deserializing std::map #1100

• What is the issue you have?
  When trying to deserialize a std::map which has a int/Enum as a key I'm getting compilation error.
  Error C2166 l-value specifies const object

When i try to deserialize a std::map with a key as a string for example, it compiles fine.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?
  Serialize a std::map having int/enum as a key. Try to deserialize it.

json.at(_materialTypeByFaceSelectionJson).get<map<MaterialTypeEnum, FaceSelectionEnum>>();

• What is the expected behavior?
  It should compile.

• And what is the actual behavior instead?
  It doesn't.

• Which compiler and operating system are you using? Is it a supported compiler?
  I'm using windows 10 and Visual Studio 2017.

• Did you use a released version of the library or the version from the develop branch?
  I'm using released version.