Move entire contentType-specific logic into RequestStrategy

[kss-espeo]

After #32 and #42 , some logic that is specific to contentType is still being done outside of RequestStrategy. This leads to code duplication, worse testability and maintainability.

In scope of this task, refactor exisiting solution to move contentType-speciic logic into RequestStrategy. This will probably mean that contentType enum will be interoduced, Request class will be made abstract and some extensions to it will be created and RequestUrlParser logic will be moved to RequestStrategy. Refer to Code Review comments in #32 and #42 .

While you're at it, it would be nice to add more tests around RequestExecutor logic.