Avoid creating new `ByteString` s

[winterland1989]

While i'm tackling #66 , i found an interesting problem: http-type would create too many ByteString:

• parseQueryText/parseQuery use urlDecode which create new ByteString s.
• HeaderItem use CI ByteString which in turn create a case-folded ByteString.

These ByteString s are not created via slice thus will be slow and create memory fragment, i would suggest to simply use Text instead.