Parse strings into dictionaries in coerce-to-dictionary

[ewpatton]

Change-Id: Ib8656bb97ce9d9bfa9cdeb711b0f2425d834a8bf

General items:

• I have updated the relevant documentation files under docs/
• My code follows the:
  • Google Java style guide (for .java files)
  • Google JavaScript style guide (for .js files)
• ant tests passes on my machine

If your code changes how something works on the device (i.e., it affects the companion):

• I branched from ucr
• My pull request has ucr as the base

Further, if you've changed the blocks language or another user-facing designer/blocks API (added a SimpleProperty, etc.):

• I have updated the corresponding version number in YaVersion.java
• I have updated the corresponding upgrader in YoungAndroidFormUpgrader.java (components only)
• I have updated the corresponding entries in versioning.js

For all other changes:

• I branched from master
• My pull request has master as the base

What does this PR accomplish?

A common pattern is that a REST call using the Web component will return JSON, but the responseContent parameter is always a string. People naturally try to pass this into a dictionary block thinking it is a dictionary, but this errors out. This PR adjusts the logic of coerce-to-dictionary so that if it is given a string it will parse it into a dictionary. We might be able to make it more efficient if we somehow cache the result of the parsing so multiple calls don't result in multiple parsing passes, but that can be an improvement left for another time.

[ewpatton]

Note that this is just the iOS implementation. We'll want to implement the equivalent logic on the Android side as well.