Fix case-sensitivity problems in extension content verification

extensions/test/data/content_verifier/README

 
 The public/private key pairs were generated with the following commands:
 
 openssl genrsa -out private_key.pem 2048
 openssl rsa -in private_key.pem -pubout -out public_key.pem
 
 
+The payload.json file contains randomly generated base64url encoded sha256
+hashes for a set of file paths.
+
+
 The signature was generated by:
 
 1) Take the contents of payload.json and base64url encode them:
 cat payload.json | tr -d \\n | base64 -w0 | tr / _ | tr + \- | tr -d '=' > payload_encoded.txt
 
 2) Put the contents of payload_encoded.txt into the "payload" field of
 verified_contents.json.
 
-3) Copy the contents of the "protected" field from verified_contents.json into
-protected.txt.
+3) Copy the contents of the "protected" field (the one with {"kid":
+"webstore"}) from verified_contents.json into protected.txt.
 
 4) Concatenate the "protected" and "payload" fields with a '.' separator.
 
 echo -n '.' | cat protected.txt - payload_encoded.txt > signature_input.txt
 
 5) Sign it
 
 tr -d \\n < signature_input.txt | openssl dgst -sha256 -sign private_key.pem -binary | base64 -w0  | tr / _ | tr + \- | tr -d '=' > signature.txt
 
 6) Put the contents of signature.txt into the "signature" field in
 verified_contents.json.