Fix iterator_input_adapter consuming more than it should

[falbrechtskirchinger]

As identified by @imwhocodes, iterator_input_adapter consumes an additional character from the input during parsing. This PR based on #3442 by @imwhocodes fixes this. I'm finishing his work by incorporating requested changes and fixing CI issues.

At the moment, it's missing a unit test.

Replaces #3442.

[coveralls]

Coverage remained the same at 100.0% when pulling dc809d4 on falbrechtskirchinger:iterator_input_adapter_fix into 48a102c on nlohmann:develop.

[falbrechtskirchinger]

I've tried to write a unit test and the "fixed" iterator_input_adapter isn't consuming enough. Either my test is flawed or there was never a problem to begin with. 😕

Parsing "[1][2]":

TEST CASE:  deserialization
  contiguous containers
  via iterator range
  verify iterator_input_adapter doesn't over-consume

/home/flo/projects/json/iterator_input_adapter_fix/tests/src/unit-deserialization.cpp:618: ERROR: CHECK( std::string(first, last) == str2 ) is NOT correct!
  values: CHECK( ][2] == [2] )

[falbrechtskirchinger]

I've now added the unit test which fails to trigger the alleged issue.

[falbrechtskirchinger]

I'm convinced this fixes nothing and breaks legitimate use cases. See #3442 for a lengthy discussion.