Bugfix/fix mask values

[khlr]

This PR fixes issue #88.

• The key comparison is now case-insensitive
• Multiple headers with same key (e.g. Set-Cookie) are now all processed

What I've done additionally:

• I have replaced the short, hard to read variable names with longer but more meaningful names. (https://github.com/simplesamlphp/SAML-tracer/commit/e2e2c0cee2afdadee29f3cd9cc5ed31724c96c64)
• I changed the behavior for boolean/flag-attributes: They are no longer considered as values but as keys, which means that they are not changed in the output. Until now, the Secure-flag, for example, produced the unattractive or "wrong" result ...9e91};={hash:a08203914d84eb};Domain=.... Instead, the output now looks like this: ...9e91};Secure;Domain=... (https://github.com/simplesamlphp/SAML-tracer/commit/27d386624298657caec79a7a839b6c151476131c)

What I didn't do:

• I didn't change anything for Firefox specifically. Which means that the "very special bahaviour" (\n, see here) remains...

[khlr]

I've just merged the PR. We'll see if there will be any changes in Firefox' webRequest-API regarding multiple headers... (see #88)