Summary
-------

Allow setting of restricted HTTP request headers through a configurable networking property.

Problem
-------

There have been a few complaints from developers needing to set HTTP request headers that are on an implementation-specific restricted list. On a number of previous occasions some of these headers have been removed from the restricted list. It makes most sense to make the list configurable so that the implementation does not have to change each time a new request is received.

Solution
--------

Create a networking property which lists the restricted headers that are to be allowed.

Specification
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Create a new networking/system property called `jdk.httpclient.allowRestrictedHeaders` which lists the restricted header names to be allowed. The property can be set in either net.properties or as a system property on the command line.

The following text is to be added to the 'net.properties' properties file

    #
    # Allow restricted HTTP request headers
    #
    # By default, the following request headers are not allowed to be set by user code
    # in HttpRequests: "connection", "content-length", "expect", "host" and "upgrade".
    # The 'jdk.httpclient.allowRestrictedHeaders' property allows one or more of these
    # headers to be specified as a comma separated list to override the default restriction.
    # The names are case-insensitive and white-space is ignored (removed before processing
    # the list). Note, this capability is mostly intended for testing and isn't expected
    # to be used in real deployments. Protocol errors or other undefined behavior is likely
    # to occur when using them. The property is not set by default.
    # Note also, that there may be other headers that are restricted from being set
    # depending on the context. This includes the "Authorization" header when the
    # relevant HttpClient has an authenticator set. These restrictions cannot be
    # overridden by this property.
    #
    # jdk.httpclient.allowRestrictedHeaders=host
    #