fixed Content-Length header

[ghost]

• It's no more possible to have a Content-Length header with NULL as value
• allow Content-Length: 0, fixes "Content-Length" header set to "0" returns header line without value #26
• added ContentLength::setFieldValue()
• throw exception on invalid Content-Type headers like Content-Type: 123.456
• restructured unit tests

[ezimuel]

Adding a setFieldValue() function is considered a new feature and it should be done with a new PR.

[ezimuel]

@DCMNMarc As I wrote in specific comments this PR should focus only on fixing #26. The other changes that you proposed should be done in a new PR as new feature. Thanks for your contribution!

Note: to fix #26 is enough to change this in constructor:

if ($value !== null) {
...

[ezimuel]

@DCMNMarc I fixed #26 issue with PR #87. Now, if you want you can focus on the new features proposed in this PR. Thanks again!

[ezimuel]

Closing this PR in favor of a new one @DCMNMarc.