- Added `CatchPanic` middleware which catches panics and converts them
  into `500 Internal Server` responses ([#214])

[#214]: #214

* Fix parsing of Accept-Encoding request header

* Add unit tests to content_encoding

* Represent quality values (qvalues) by a separate type

* Parse encodings case-insensitively

* Parse qvalues as specified in RFC 7231 section 5.3.1

Refs: #215

* Do not use or-pattern syntax

This syntax is not supported in rust 1.51 (the minimum toolchain
version).

* Add comments to QValue::parse

* Remove redundant SupportedEncodingsAll::new function

* Add unit tests for all content-encodings (gzip, deflate, br)