Keep a consistent Content Type value for one ADLS Gen2 folder between Editor and Properties dialog

Storage Explorer Version: 1.13.0
Build: 20200422.4
Branch: rel/1.13.0
Platform/OS: Windows 10/ Linux Ubuntu 18.04/macOS High Sierra
Architecture: ia32/x64
Regression From: Not a regression

Steps to reproduce:

1. Expand one ADLS Gen2 storage account -> Blob Containers.
2. Create a blob container -> Upload one folder.
3. Right click the folder -> Click 'Properties...'.
4. Check the Content type on properties dialog.

Expect Experience:
Display a consistent Content Type value between Editor and Properties dialog.

Actual Experience:

1. Display an inconsistent Content Type value between Editor and Properties dialog.
2. The ContentType value is null in Properties dialog.
   
3. Open the Properties dialog -> Click 'Save' -> Re-open the Properties dialog.
   The ContentType value is changed to 'application/octet-stream'.
   

More Info:

For one new created ADLS Gen2 folder

1. The ContentType value is 'application/json; charset=utf-8'.
   
2. Open the Properties dialog -> Delete the current ContentType value -> Click Save -> Re-open the Properties dialog. The ContentType value is changed to 'application/octet-stream'.
   

Start by reproducing the issue with an ADLS Gen2 folder and compare the Content Type shown in the Editor and Properties dialog. Trace how the Properties dialog reads and saves ContentType, then verify that reopening it preserves the same value without changing it to null or application/octet-stream.