Throw exception when field type is incorrect

[outdooracorn]

Another late addition to our improvements on error handling (#53).

Fixes the following edge cases:

• if the "op" field is boolean true, it is interpreted as an "add" operation instead of throwing an exception. This is due to true always matching the first [switch case] (Add::OP).
• if the "op" field is an Array (list) or Object, it results in a "PHP Notice Array/Object to string conversion" in the [UnknownOperationException]
• if the "path" or "from" fields are not a string, an exception will be thrown in JsonPatch::apply() rather than JsonPatch::import()

[codecov[bot]]

Codecov Report

Merging #60 (f8fcc60) into master (b9da3c5) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   96.85%   96.95%   +0.09%     
==========================================
  Files          14       15       +1     
  Lines         573      591      +18     
==========================================
+ Hits          555      573      +18     
  Misses         18       18              

Impacted Files	Coverage Δ
src/InvalidFieldTypeException.php	100.00% <100.00%> (ø)
src/JsonPatch.php	99.06% <100.00%> (+0.05%)	:arrow_up:

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[outdooracorn]

Realised I didn't explain what this PR addresses! :see_no_evil:

We noticed the following edge cases:

• if the "op" field is boolean true, it is interpreted as an "add" operation instead of throwing an exception. This is due to true always matching the first switch case (Add::OP).
• if the "op" field is an Array (list) or Object, it results in a "PHP Notice Array/Object to string conversion" in the UnknownOperationException
• if the "path" or "from" fields are not a string, an exception will be thrown in JsonPatch::apply() rather than JsonPatch::import()

The second commit, makes sure test code coverage doesn't decrease. I suggest we ignore the code climate issue. :)