Add new MapELResolver with type coercion to support accessing enum keys

[liamrharwood]

related to #271

This PR adds the new TypeConvertingMapELResolver in order to allow accessing map entries when their key does not have equality with its string representation.

This is most often seen with enums. For instance, imagine a Java map with key type MyEnum:

public enum MyEnum { FOO, BAR; } 

Outputting the map via {{ map }} could produce the following:

{FOO=value1, BAR=value2}

However, {{ map.FOO }} would not resolve to anything as the string "FOO" does not have equality with MyEnum.FOO.

To fix this, I've used the CollectionMembershipOperator and TruthyTypeConverter to determine if a key is valid and convert it to the correct class. This means that this should work for all types that have this issue, not just enums (for instance, numeric types).

After this PR, {{ map.FOO }} will output value1, which is the expected result.

[liamrharwood]

@mattcoley Since jinjava actually uses the toString representation when printing enums, Enum.valueOf in the regular coerceToEnum will not always match. For that reason, I added extra logic here to check the toString values of the enums as well.
You can see an example in the itGetsDictValuesWithEnumKeysUsingToString test case below.