buffer_copy test expansion

[cmazakas]

buffers/test/buffer_copy.cpp

Lines 76 to 77 in 483c6c7

	void
	testEmptyBufferCopy()

These tests need to be expanded to cover a few more use-cases.

They should include using buffers that are default-constructed and buffers constructed from a default-constructed string_view.

For example:

const_buffer source;
// and
boost::core::string_view sv;
const_buffer source(sv);