mockResponse - add support for Buffer writes, tests

[LoneRifle]

From @cpd0101:

• Allow encoding to be retrieved via getEncoding
• Support the use of Buffers when calling write/end

From myself:

• Declare a new getter _getChunks, for testing purposes
• Add javadoc for _getBuffer
• Add a suite of tests that ensure that calls to write and end with payloads result in the correct state of _data, _chunks and _buffer. Ensure that methods are tested in isolation, using a separate suite for interactions between write and end

This PR builds on and supersedes #154

[LoneRifle]

No longer supersedes #154