Fix object strong guarantee

[grisumbras]

Fix #763

[vinniefalco]

There is no requirement that capacity is restored to the previous value on an exception.. it is ok to increase capacity and then later fail the insertion - and the object is returned to the caller with the increased capacity.

[grisumbras]

If new storage is allocated, then iterators are invalidated and thus, the function doesn't provide the strong guarantee.

[grisumbras]

It's not too hard to revert storage change in our case, so I don't see why we shouldn't. Strong guarantee is a good property.

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/907/pullrequest.html

[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/907/pullrequest.html

[codecov]

Codecov Report

Merging #907 (8957955) into develop (4700500) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #907   +/-   ##
========================================
  Coverage    92.98%   92.98%           
========================================
  Files           85       85           
  Lines         8056     8058    +2     
========================================
+ Hits          7491     7493    +2     
  Misses         565      565           

Impacted Files	Coverage Δ
include/boost/json/object.hpp	100.00% <ø> (ø)
include/boost/json/impl/object.hpp	100.00% <100.00%> (ø)
include/boost/json/impl/object.ipp	100.00% <100.00%> (ø)

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/907/pullrequest.html