[XrdHttp] Don't add blank header if m_digest_header is empty

[jthiltges]

Testing 4.9.1-rc2, @matyasselmeci noted that HTTP output prepends a \r\n to the expected contents.

@bbockelm was able to isolate the issue and suggested the patch in this PR.

This would be helpful to have in RC3. @simonmichal

Example:
xrootd-4.9.1-0.1.rc1.osg34.el7.x86_64

$ curl -s -k -E /tmp/cert.pem 'https://xrootd.example.edu:1094/hello_world.txt' | hexdump -C
00000000  68 65 6c 6c 6f 20 77 6f  72 6c 64 21 0a           |hello world!.|
0000000d

xrootd-4.9.1-0.2.rc2.osg34.el7.x86_64

$ curl -s -k -E /tmp/cert.pem 'https://xrootd.example.edu:1094/hello_world.txt' | hexdump -C
00000000  0d 0a 68 65 6c 6c 6f 20  77 6f 72 6c 64           |..hello world|
0000000d