inject: check first if Vue prototype key exists.

[phlegx]

What kind of change does this PR introduce?

Function inject uses prototype for Vue to inject a new object without to check if the key already exists.

What is the current behavior?

No property check before inject new object. Results in an error.

What is the new behavior?

Function inject tests if the key already exists in Vue. If the key exists, adding is skipped for Vue object.

Checklist:

• [x] Documentation
• [x] Tests