Folders containing Markdown Note documents should be downloadable as ZIP

If a folder or workspace contains a Markdown Note document, it cannot be downloaded.

Steps to reproduce:

• Create a Folder or Workspace document.
• Within the Folder or Workspace document, create a Note document of type Markdown.
• Add the following content to the Markdown Note:

  # H1 Heading
  
  ## H2 Heading
  
  Some text here.
  
  **Some bold   text.**
  
  *Some italics.*
  
  Some bullet points:
  * one
  * two
  * three
  

• Save.
• Navigate to the parent folder/workspace.
• From the Document Actions, select Download as ZIP.
  => ZIP not downloaded.

Toaster notification:

An error occurred while zipping files:
Error: missing Content-Disposition

(the rest of the error message is cut off)

Error in console:

Uncaught (in promise) Error: missing Content-Disposition header
    at HTMLElement._download (vendors~elements.bundle.js:758)
    at vendors~elements.bundle.js:758

Error in server log:

2019-03-26T09:37:42,919 WARN  [Nuxeo-AsyncOperation-50caaaf6-f382-4ad3-b5e7-a31fd758b2d7] [org.nuxeo.ecm.automation.core.trace.Trace] 

****** chain ******
Name: blobToPDF
Exception: OperationException
Caught error: Failed to invoke operation Blob.ToPDF
Caused by: org.nuxeo.ecm.automation.OperationException: Failed to invoke operation Blob.ToPDF
****** Hierarchy calls ******
    org.nuxeo.ecm.automation.core.operations.stack.PopBlob
        org.nuxeo.ecm.automation.core.operations.blob.BlobToPDF