Update cache key for stat var search POSTs (#4537)

Quick fix to follow up
#4534 and unblock the
website release. For the future it would be nice to wrap @cache.cached
in a custom decorator that we can use everywhere which takes care of
passing default params and making sure post body is in the cache key.

server/routes/shared_api/stats.py

@@ -22,6 +22,7 @@
 from server.lib import fetch
 from server.lib import shared
 from server.lib.cache import cache
+import server.lib.util as lib_util
 from server.routes import TIMEOUT
 import server.services.datacommons as dc
 
@@ -99,7 +100,9 @@ def stat_var_property():
 
 
 @bp.route('/stat-var-search', methods=('GET', 'POST'))
-@cache.cached(timeout=TIMEOUT, query_string=True)
+@cache.cached(timeout=TIMEOUT,
+              query_string=True,
+              make_cache_key=lib_util.post_body_cache_key)
 def search_statvar():
   """Gets the statvars and statvar groups that match the tokens in the query."""
   if request.method == 'GET':