Add to default headers at time of initialization

[vernak2539]

Give ability to a global default header, which is passed in at the time of init.

Test to pass (should be added to general-tests.js)

it('should add header from options to global headers (this.defaultHeaders)', function() {
    // Arrange
    var client;
    initOptions.headers = { newHeader: true };

    // Act
    client = new FuelSoap(initOptions);

    // Assert
    assert.ok(client.defaultHeaders.newHeader);
});