This is required to avoid creation of temporary ImmutableString
like so:

std::string x;
ImmutableString y = ...;
StringRef ref = !x.empty() ? x : y;

First, temporary ImmutableString is created with x since
ImmutableString has constructor to accept std::string.  After
StringRef gets this, the temporary ImmutableString is destroyed, and
ref has dangling pointer.

This might help throughput, but it interfere stream priority.  The
throughput issue is generally caused by the small buffer size to store
response body, which was 16K.  We increased it to 128K to compensate
this change.

Because of zero initialization, these assignments are unnecessary.

We won't add operator[] to StringRef.  This is because it may be
undefined if pos == size(), and StringRef's base + len does not point
to the valid region.  This solely depends on the given buffer, so we
cannot do anything to fix.  For workaround, if we need this kind of
operator, we may add it under another name, like char_at(size_type).