fix: handle 0-length header values and trim whitespace

[patrickhulce]

5556015 seems to have broken the tests & handling of whitespace

with the way the regex is now it will never match anything since \s is part of the character class in the next group, so from what I can tell we can either remove \s*? entirely to not trim whitespace, throw or .trim on the value, or do something like I've done here

also added a test case for #60

[developit]

beautiful