Add fix when flexible content return something diferent to String.

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Fix when the flexible content return one bolean element
What is the current behavior? (You can also link to an open issue here) When return one bolean element don't show the content of the page
What is the new behavior (if this is a feature change)? Convert the bolean to string and fix the problem
Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
Other information:

moxie-lean / ng-patternlab