Fix #1217 - converted some properties from implicitly unwrapped optionals to address crash

[jvisenti]

This PR changes JTACMonthLayout.delegate and JTACMonthDelegateProtocol._cachedConfiguration from implicitly unwrapped options to optionals. It addresses a crash reported in #1217 where the following line crashes in rare circumstances due to a force unwrapped optional with a nil value.

JTACMonthLayout.Swift:168:

strictBoundaryRulesShouldApply = thereAreHeaders || delegate._cachedConfiguration.hasStrictBoundaries

Although I don't know under which circumstances the implicitly unwrapped optional(s) are nil, this PR should at least prevent the crash. It should have no impact on functionality.

[mohanvydehi]

@patchthecode Is there any plans to merge this PR ? I'm facing the exact same crash in my code.

[patchthecode]

I'll review this on the weekend. in the mean time @mohanvydehi does this fix your issue?

[mohanreddys-wal]

Yes, it worked for me @patchthecode

[robertcrabtree]

Our team has the same issue. It would be great to see this PR merged.