Resolves parameter binding error when task is auto-updated

[jessehouwing]

Fixes #12

Workaround:

• Set to "External" enter a value in "Git source Url", save the definition.
• Set back to github, save the definition.

Alternate workaround

• delete task from current pipeline and add again to have the default value (null) be populated.

Caused by:

• No default value provided for Git Source Url, causing funny behavior.

---

This change is 

[jakobehn]

I just did similar changes and pushedd a new version (see #12). I changed the GitSourceUrl parameter to be optional, since the default is GitHub

[jessehouwing]

That works too :). I still recommend to put in a UI level default value. Even if it's empty-string. It makes upgrading easier in the long run.

[jakobehn]

Closing this since the feature is implemented through #12