Don't do merge if there is nothing to merge

[Earendil95]

Hey Dmitry!

Thanks for this gem :)

I've noticed one minor possible improvement. In ArLazyPreload::Merger before choosing a merging strategy, we check if we need to merge something. The thing is: other.lazy_preload_values is always true when it comes to if other.lazy_preload_values. This unexpected merging provoked some errors, pretty similar to one discussed in #30. Of course, this PR is not going to fix the real issue (possible dead associations) but it can prevent people from catching weird errors right after including ar_lazy_preload to Gemfile from unexpected places and blaming ar_lazy_preload (as it appears in the very top of backtrace).

PS. I didn't find a way to test this improvement. If you have ideas about it, please, share them and I will add a test here.

[coveralls]

Pull Request Test Coverage Report for Build 243

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.287%

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Totals
Change from base Build 241:	0.0%
Covered Lines:	647
Relevant Lines:	679

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💛 - Coveralls

[PikachuEXE]

I think you can post the benchmark like I did in #16

[Earendil95]

@PikachuEXE thanks for the comment

To be honest, I don't think a benchmark is valuable here because no big optimizations have been done here, only error-proof.

[DmitryTsepelev]

Agreed, it won't affect benchmarks, but the check definitely makes sense. Thank you so much!

[Earendil95]

@DmitryTsepelev thanks a lot for your quick reaction!