transport: fix Content-Type for pprof data

[axw]

Oops, copy-pasta.

[codecov-io]

Codecov Report

Merging #679 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #679      +/-   ##
==========================================
+ Coverage   87.72%   87.77%   +0.05%     
==========================================
  Files         125      125              
  Lines        7893     7893              
==========================================
+ Hits         6924     6928       +4     
+ Misses        861      857       -4     
  Partials      108      108

Impacted Files	Coverage Δ
transport/http.go	92.42% <100%> (ø)	⬆️
profiling.go	86.88% <0%> (+6.55%)	⬆️

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