Define a stable way to serialize types

Current implementation of DefaultParameterizedType#toString produces output such as:

java.util.List< java.lang.String >

those spaces are unnecessary and not present in the implementation of ParameterizedTypeImpl of Java: https://github.com/frohoff/jdk8u-jdk/blob/master/src/share/classes/sun/reflect/generics/reflectiveObjects/ParameterizedTypeImpl.java#L229

We should provide a standard way to serialize all types to avoid this kind of inconsistencies.