Switch from an assert to a throws

Author: joegallo

server/src/main/java/org/elasticsearch/ingest/IngestDocMetadata.java

@@ -51,7 +51,9 @@ class IngestDocMetadata extends Metadata {
 
  private static Map<String, FieldProperty<?>> validateLeadingUnderscores(final Map<String, FieldProperty<?>> properties) {
  for (String key : properties.keySet()) {
- assert key.charAt(0) == UNDERSCORE;
+ if (key.charAt(0) != UNDERSCORE) {
+ throw new IllegalArgumentException("IngestDocMetadata keys must begin with an underscore, but found [" + key + "]");
+ }
  }
  return properties;
  }