Fix broken `Tracer.configure(disabled?: true)`

[sekiyama58]

Currently, even when Tracer.configure(disabled?: true) is called, the spandex is not disabled.

There is another issue that calls of trace_start and span_start after dynamic configuration of tracer using Tracer.configure(service: :app, adapter: ..., disabled?: true) will cause exception like:

    ** (EXIT) an exception was raised:
        ** (ArgumentError) Opt Validation Error: service - is required, adapter - is required
            (optimal) lib/optimal.ex:44: Optimal.validate!/2
            (mikoto) lib/mikoto/tracer.ex:2: Mikoto.Tracer.config/2
            (mikoto) lib/mikoto/tracer.ex:2: Mikoto.Tracer.start_span/2

This is because the option is not Application.put_env when disabled?: true.

This patch fixes the issue.

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[coveralls]

Pull Request Test Coverage Report for Build 312

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.8%) to 83.628%

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Totals
Change from base Build 311:	1.8%
Covered Lines:	189
Relevant Lines:	226

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💛 - Coveralls

[zachdaniel]

Thanks for your contribution! Additionally, thanks for including a test to validate your change. Much appreciated.