Use `const` instead of `static`
Merge pull request #74 from s-panferov/fix/use-const

Use `const` instead of `static`
fix client requests dropping the query string. fixes #75
Merge pull request #76 from hyperium/fix-query

fix client requests dropping the query string. fixes #75
move rendering of headers to the Headers object
Slice was changed to AsSlice in libstd
Merge pull request #78 from hyperium/headers-fmt

Minor internal cleanup of header formatting