Integer Overflow in HeaderMap::reserve() can cause Denial of Service

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.