Fix output of Object.toString()

[RageKnify]

This Pull Request fixes/closes #729.

It changes the following:

• Object::to_stringreturns a Value::String respecting the spec: https://tc39.es/ecma262/#sec-object.prototype.tostring

[codecov]

Codecov Report

Merging #737 into master will increase coverage by 0.05%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##           master     #737      +/-   ##
==========================================
+ Coverage   71.75%   71.80%   +0.05%     
==========================================
  Files         200      200              
  Lines       13983    14012      +29     
==========================================
+ Hits        10033    10062      +29     
  Misses       3950     3950              

Impacted Files	Coverage Δ
boa/src/builtins/object/mod.rs	50.49% <50.00%> (-1.32%)	⬇️
boa/src/builtins/object/tests.rs	100.00% <100.00%> (ø)
tester/src/main.rs
tester/src/exec.rs
tester/src/results.rs
tester/src/read.rs
boa_tester/src/results.rs	0.00% <0.00%> (ø)
boa_tester/src/exec.rs	0.00% <0.00%> (ø)
boa_tester/src/main.rs	0.00% <0.00%> (ø)
boa_tester/src/read.rs	0.00% <0.00%> (ø)
... and 2 more

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[Razican]

I like this approach, but now that we are at it, I think we should implement it to be spec compliant:
https://tc39.es/ecma262/#sec-object.prototype.tostring

[RageKnify]

I like this approach, but now that we are at it, I think we should implement it to be spec compliant:
https://tc39.es/ecma262/#sec-object.prototype.tostring

Yeah, my bad to not look for it.

[RageKnify]

I assume one of the bugs in tarpaulin makes it so lines 129 through 136 in boa/src/builtins/object/mod.rs are considered not covered since they are covered by the introduced test.

[Razican]

Looks good from my side! Thanks!

[RageKnify]

Output for [object Error] is currently not tested, I think I'll include it in #749 where I'll introduce the changes to the error constructors discussed in Discord.