fix: improve responseHasBody check for responses with an unknown size

[jely2002]

Description

Changes the responseHasBody check, so that in case of a null or -1 for the size value we peek at the content to confirm if we have a body or not.

Motivation and context

With this change a body size of -1/null no longer evaluates to the response having a body. Meaning responses with a -1 for the size get a proper check.

Closes #110

How has this been tested?

I've added 2 tests that check if the new piece of code can detect empty/filled responses when the size is unknown.
I think this won't have many more side effects in the rest of the code.

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[JaZo]

Shouldn't we read just a few bytes instead of the entire stream?