http_server: save 450+ bytes of memory on x86-64

Replacing the Regexp argument to a rarely-called String#split with a
literal String can save a little memory.  The removed Regexp memsize
is 469 bytes on Ruby 2.1:

	ObjectSpace.memsize_of(/,/) => 469

Is slightly smaller at 453 bytes on 2.2.0dev (r48474).
These numbers do not include the 40-byte object overhead.

Nevertheless, this is a waste for non-performance-critical code
during the socket inheritance phase.  A literal string has less
overhead at 88 bytes:

	* 48 bytes for table entry in the frozen string table
	* 40 bytes for the object itself

The downside of using a literal string for the String#split argument
is a 40-byte string object gets allocated on every call, but this
piece of code is only called once in a process lifetime.