Fixes for CORS implementation

[julienr]

This fixes two issues with the CORS implementation in azurite.

Without this, CORS doesn't work when testing on azurite even though it works when testing on the live blobstore. I'm using azurite master on OSX with node v8.4.0 .

The first issue can be reproduced by using the script below which will result in a failed destructuring assignment “TypeError: Cannot match against 'undefined' or 'null’.”

# Create the container
#az storage container create \
#    --name 'userblobs' \
#    --connection-string 'DefaultEndpointsProtocol=http;AccountName=devstoreaccount1;AccountKey=Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==;BlobEndpoint=http://127.0.0.1:10000/devstoreaccount1;'


CORS_XML="
<?xml version="1.0" encoding="utf-8"?>  
<StorageServiceProperties> 
    <Cors>  
        <CorsRule>  
            <AllowedOrigins>*</AllowedOrigins>  
            <AllowedMethods>PUT,POST,GET,HEAD,OPTIONS</AllowedMethods>  
            <MaxAgeInSeconds>200</MaxAgeInSeconds>  
            <ExposedHeaders>*</ExposedHeaders>  
            <AllowedHeaders>*</AllowedHeaders>  
        </CorsRule>  
    </Cors> 
</StorageServiceProperties> 
"

curl -X PUT \
    -H 'Accept: application/xml' \
    -H 'Content-Type: application/xml' \
    -d "$CORS_XML" \
    http://127.0.0.1:10000/devstoreaccount1?restype=service\&comp=properties


curl -H "Origin: http://localhost:8080" \
     -H "Access-Control-Request-Method: PUT" \
     -H "Access-Control-Request-Headers: x-ms-blob-content-disposition,x-ms-blob-type" \
     -X OPTIONS --verbose \
     http://127.0.0.1:10000/devstoreaccount1/userblobs/rasters/387dcec91bd8493ba9b8cd4cc3bb521d/example_1.tif\?se\=2018-03-17T16%3A22%3A51Z\&sp\=w\&sv\=2017-07-29\&sr\=b\&sig\=YhIBWOVv/G4XfWnowBVG9U2oLUeIbIcBqm9c7b0wzew%3D

The second issue can only be reproduced when testing CORS from a browser and is due to the fact that azurite wouldn't set the Access-Control-Allow-Headers on its response to the preflight request, making the browser believe CORS is disallowed.

[arafato]

@julienr this is really awesome! Thanks a lot for the fix. Will release as part of v2.6.2.

[julienr]

Thanks for the quick merge !