Set altered HTTPRequest attributes as parameters.
test that invalid unicode doesn't raise 500
Merge pull request #945 from chrismcguire/request_body_property

Add properties corresponding to HTTPRequest constructor arguments
rase 400, not 500 on invalid unicode in decode_argument
Remove new HTTPRequest.if_modified_since property.

This property was unusual in that its getter returned a value its
setter would not accept, and was redundant with direct access to the
headers.

Use the property setters in the constructor instead of duplicating logic.
expect log message in test_decode_argument_invalid_unicode
Merge pull request #963 from minrk/invalid-unicode

bad unicode in URLs shouldn't cause 500
Improve log message for invalid unicode.

Truncate long values, and include the argument name if available.