Improve creation performance of HttpIncoming

[trygve-lie]

HttpIncoming lives in hot path of every request. This PR doubles the creation performance of HttpIncoming.

From:

new HttpIncoming() - No params x 91,806 ops/sec ±2.48% (85 runs sampled)
new HttpIncoming() - With params x 92,816 ops/sec ±2.34% (85 runs sampled)

to:

new HttpIncoming() - No params x 186,691 ops/sec ±1.04% (90 runs sampled)
new HttpIncoming() - With params x 186,027 ops/sec ±1.54% (92 runs sampled)

There is probably more performance to press out of this but that involves taking a stab on the original-url module.