Honor TZID when evaluating UNTIL [Issue 37]

[Ferix9288]

Issue: https://github.com/teambition/rrule-go/issues/37

Easy fix where I call StrToROptionInLocation with defaultLoc instead of just StrToROption that defaults to UTC.

        case "RRULE", "EXRULE":
            rOpt, err := StrToROptionInLocation(rule, defaultLoc)
            if err != nil {
                return nil, fmt.Errorf("StrToROption failed: %v", err)
            }

Other notable changelog:

• Updated TestStrSet and TestRFCSet so that the UNTIL now respects UTC or NY timezone
• Added a testcase and imported assert and require library. I'm ok with rejecting this added testcase.

[coveralls]

Coverage remained the same at 99.663% when pulling 3c634b3a36fd6fdbea07c53d4897bcb8d8562cd0 on Ferix9288:fix_issue_37 into c4b1bf2c7bbb37a14f9614eaf0b5d764fc173269 on teambition:master.

[Ferix9288]

Never mind. Looks like this is intentional behavior based on the RFC spec.