http known bugs

HeaderMap::reserve() used usize::next_power_of_two() to calculate the increased capacity. However, next_power_of_two() silently overflows to 0 if given a sufficiently large number in release mode. If the map was not empty when the overflow happens, the library will invoke self.grow(0) and start infinite probing. This allows an attacker who controls the argument to reserve() to cause a potential denial of service (DoS). The flaw was corrected in 0.1.20 release of http crate.

Affected versions of this crate incorrectly used raw pointer, which introduced unsoundness in its public safe API. [Failing to drop the Drain struct causes double-free](https://github.com/hyperium/http/issues/354), and [it is possible to violate Rust's alias rule and cause data race with Drain's Iterator implementation](https://github.com/hyperium/http/issues/355). The flaw was corrected in 0.1.20 release of `http` crate.

`HeaderMap::reserve()` used `usize::next_power_of_two()` to calculate the increased capacity. However, `next_power_of_two()` silently overflows to 0 if given a sufficiently large number in release mode. If the map was not empty when the overflow happens, the library will invoke `self.grow(0)` and start infinite probing. This allows an attacker who controls the argument to `reserve()` to cause a potential denial of service (DoS). The flaw was corrected in 0.1.20 release of `http` crate.

An issue was discovered in the http crate before 0.1.20 for Rust. The HeaderMap::Drain API can use a raw pointer, defeating soundness.