Error testing existence of a directory

### Which service(blob, file, queue, table) does this issue concern?
Blob

### Which version of the Azurite was used?
Built the 3.4.0 docker container from commit

### Where do you get Azurite? (npm, DockerHub, NuGet, Visual Studio Code Extension)
Github

### What's the Node.js version?

6.13.4

### What problem was encountered?
I use the [azure-storage-blob](https://github.com/Azure/azure-storage-python) Python library to test for the existence of a "directory". Directories are referred to as BlobPrefix.

The `.exists()` method makes the following API call

`"HEAD /devstoreaccount1/files/text/ HTTP/1.1" 404 -`

Testing this on real Azure accounts the HEAD will return 202 for a valid prefix.

### Steps to reproduce the issue?

`conda create -n python=3.7 azure-storage-blob=2.1.0 azure-storage-common=2.1.0`

Run the following code

```python
from azure.storage.blob import BlockBlobService

bbs = BlockBlobService(is_emulated=True)

# create container, prefix, and blob
bbs.create_container('files')
bbs.create_blob_from_text('files', 'text/hello.txt', 'Hello, World!')

# identify the prefix
entries = [(type(b),b.name) for b in bbs.list_blobs('files', prefix='text', delimiter='/')]
print(entries)

# identify the blob
entries = [(type(b),b.name) for b in bbs.list_blobs('files', prefix='text/', delimiter='/')]
print(entries)

print(f'prefix text/ exists: {bbs.exists("files", "text/")}')
print(f'blob text/hello.txt exists: {bbs.exists("files", "text/hello.txt")}')
```

With Azurite the following output is produced

```
[(, 'text/')]
[(, 'text/hello.txt')]
prefix text/ exists: False
blob text/hello.txt exists: True
```

### Have you found a mitigation/solution?
I had a colleague test the `.exists()` function. I am setting up a real Azure account to test this myself.