Fixing the colorbar title for colorbars on the top.

[zhiyuanzhai]

Related: #4183.

For the following test code:

using Plots
pgfplotsx()
plot(rand(10, 2), marker_z=rand(10), seriestype=:scatter, cb=:top, cb_title="mark")

before this fix:

while after this fix:

[codecov]

Codecov Report

Merging #4191 (217f629) into master (0622042) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4191      +/-   ##
==========================================
- Coverage   77.83%   77.82%   -0.02%     
==========================================
  Files          28       28              
  Lines        7056     7057       +1     
==========================================
  Hits         5492     5492              
- Misses       1564     1565       +1     

Impacted Files	Coverage Δ
src/backends/pgfplotsx.jl	71.13% <50.00%> (-0.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0622042...217f629. Read the comment docs.

[BeastyBlacksmith]

Thanks!