Add the ability to send an action containing several headers with the same name

[jaunis]

The library does not allow sending an action containing several headers with the same name, although this is required for some specific actions (the Variable header in MessageSend for instance).

This pull request adds this ability without changing the usual action semantics. A new function multivalAction() instantiates a special class which allows specifying several times the same header.

Example:

var action = client.multivalAction('MyAction');
action.MyHeader = 'myvalue';
action.MyHeader = 'another_value';

will result in:

Action: MyAction
MyHeader: myvalue
MyHeader: another_value

[coveralls]

Coverage increased (+0.2%) to 97.222% when pulling 2114629 on jaunis:master into 674e88d on BelirafoN:master.