Reduce allocations from HttpString inn various places

Hi,

during profiling I found that some allocations are caused by `HttpString` that could be avoided.

Most notably:

• HttpTransferEncoding.persistentConnection
• ServletInitialHandler.isMultipartExchange

The former is a bit unfortunate as the method already uses an optimization for HTTP1/1 but not for HTTP1/0 (which unfortunately the command line tool `ab` is using.)

I'll provide a PR shortly for these cases.

Overall, these allocations make up for 2-3% of the benchmarks. Not huge, but noticeable and - most importantly - easy to avoid.