[Backport 2.8.x][#3850] Fix the problem when setting content-type by ResponseEntity.header() does not take effect

[Sayaka617]

(cherry picked from commit c57f5cf)

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[codecov-commenter]

Codecov Report

Merging #3856 (37e268e) into 2.8.x (45910ef) will increase coverage by 0.01%.
The diff coverage is 81.81%.

❗ Current head 37e268e differs from pull request most recent head 5272af4. Consider uploading reports for the commit 5272af4 to get more accurate results

@@             Coverage Diff              @@
##              2.8.x    #3856      +/-   ##
============================================
+ Coverage     74.41%   74.42%   +0.01%     
- Complexity      689      690       +1     
============================================
  Files          1592     1592              
  Lines         40003    40013      +10     
  Branches       3653     3654       +1     
============================================
+ Hits          29769    29781      +12     
+ Misses         8744     8741       -3     
- Partials       1490     1491       +1     

Impacted Files	Coverage Δ
...rvicecomb/foundation/vertx/http/DownloadUtils.java	71.42% <33.33%> (-7.52%)	⬇️
...comb/demo/springmvc/client/TestDownloadSchema.java	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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