Check if file in mem cache exists

[renefloor]

Could fix #135

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix

⤵️ What is the current behavior?

Currently the store doesn't check if a file exists when it has a CacheObject in memory.
When the file is removed by the file system the store still returns it as though the file does exist.

🆕 What is the new behavior (if this is a feature change)?

When return the object in memory it checks if the file exists and doesn't return it otherwise.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

I tested the example app with the flow in #135 (comment)

In the old situation I could clear the cache and the app still says it loads the file from cache. In the new situation it downloads the file again.

📝 Links to relevant issues/docs

Fixes #135

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[codecov]

Codecov Report

Merging #190 into develop will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #190      +/-   ##
===========================================
+ Coverage    73.28%   73.34%   +0.06%     
===========================================
  Files           13       13              
  Lines          423      424       +1     
===========================================
+ Hits           310      311       +1     
  Misses         113      113              

Impacted Files	Coverage Δ
flutter_cache_manager/lib/src/cache_store.dart	88.04% <100.00%> (+0.13%)	⬆️

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