DefaultSingleAddressHttpClient may add incorrect host header by default

[idelpivnitskiy]

Motivation:

Existing approach for adding HostHeaderHttpRequesterFilter in
DefaultSingleAddressHttpClientBuilder keeps reference to the
hostToCharSequenceFunction at the time of class initialization. However,
hostToCharSequenceFunction can be changed later (in a proxy ctor or
unresolvedAddressToHost builder method). In these cases,
hostHeaderFilterFactoryFunction keeps the reference to the old
hostToCharSequenceFunction.

Modifications:

• Use a boolean to know when HostHeaderHttpRequesterFilter should be
  applied;
• When HostHeaderHttpRequesterFilter is added, use the latest reference to
  the hostToCharSequenceFunction;

Result:

HostHeaderHttpRequesterFilter uses the latest version of
hostToCharSequenceFunction to determine the fallback value for the host
header.