Do not drop legacy profile alloc metrics when they are equal to in-use ones

[aalexand]

Fixes #432.

In #432 the user has a number of collected heap profiles in the legacy format that they ask pprof to merge and open and the merge fails.

The reason for the error is that one of the profiles has the alloc metric values equal to the in-use ones in the header line. Since pprof legacy profile parser drops the alloc metrics in this case, the profiles can't be merged since the merge requires that the sample types are the same between merged profiles.

This PR makes the parser stop dropping the alloc metrics in this case. This behavior seems more correct anyway as there doesn't seem to be a strong reason for such implicit behavior and in case of parsing profile.proto profiles there isn't such implicit behavior either. But may be I am missing some history here, so please review.

[codecov-io]

Codecov Report

Merging #433 into master will increase coverage by 0.05%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   67.09%   67.14%   +0.05%     
==========================================
  Files          37       37              
  Lines        7594     7594              
==========================================
+ Hits         5095     5099       +4     
+ Misses       2096     2093       -3     
+ Partials      403      402       -1

Impacted Files	Coverage Δ
profile/legacy_profile.go	77.31% <100%> (+0.67%)	:arrow_up:

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