Fix SFTPClient.mkdirs to not inadvertently prefix with '/'

[hierynomus]

@dkocher Could I ask you to verify whether this does not break in Cyberduck. The old sftp testcases were somewhat limited, so I hope you have some additional cases ;)

[codecov-io]

Codecov Report

Merging #415 into master will increase coverage by 0.21%.
The diff coverage is 94.11%.

@@             Coverage Diff             @@
##             master    #415      +/-   ##
===========================================
+ Coverage     55.28%   55.5%   +0.21%     
- Complexity     1182    1189       +7     
===========================================
  Files           192     192              
  Lines          7776    7785       +9     
  Branches        702     704       +2     
===========================================
+ Hits           4299    4321      +22     
+ Misses         3127    3115      -12     
+ Partials        350     349       -1

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/net/schmizz/sshj/sftp/PathComponents.java	85% <100%> (+20.71%)	11 <4> (+4)	⬆️
...rc/main/java/net/schmizz/sshj/sftp/PathHelper.java	82.6% <87.5%> (+2.6%)	12 <7> (ø)	⬇️
src/main/java/net/schmizz/sshj/SSHClient.java	58.28% <0%> (-0.62%)	32% <0%> (-1%)
...zz/sshj/connection/channel/ChannelInputStream.java	76.11% <0%> (+2.98%)	16% <0%> (+1%)	⬆️
...rc/main/java/net/schmizz/sshj/sftp/SFTPEngine.java	59.45% <0%> (+3.6%)	23% <0%> (+1%)	⬆️
...rc/main/java/net/schmizz/sshj/sftp/SFTPClient.java	33.78% <0%> (+5.4%)	9% <0%> (+1%)	⬆️
...t/schmizz/sshj/connection/ConnectionException.java	44.44% <0%> (+11.11%)	4% <0%> (+1%)	⬆️

---

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 7556a7f...2227683. Read the comment docs.