Use field names instead of field titles in "Issue" class

[aik099]

Problem

Right now the chobie\Jira\Issue class using field titles, when accessing fields in getter methods. This would end up badly, when field titles are changed on a JIRA installation OR an issue field automapping is turned off.

I recommend using JIRA field keys, which are immutable instead.

Solution

The getter methods of the "Issue" class (e.g. "getSummary", "getIssueType") to fallback to issue field ids (e.g. "issuetype" instead of "Issue Type"), when automapped field isn't found.