force collection_date to be a string

tasks/utilities/submission/task_submission.wdl

@@ -54,7 +54,7 @@ task prune_table {
 
     # read export table into pandas
     tablename = "~{table_name}-data.tsv"
-    table = pd.read_csv(tablename, delimiter='\t', header=0, dtype={"~{table_name}_id": 'str'}) # ensure sample_id is always a string)
+    table = pd.read_csv(tablename, delimiter='\t', header=0, dtype={"~{table_name}_id": 'str', "collection_date": 'str'}) # ensure sample_id is always a string)
 
     # extract the samples for upload from the entire table
     table = table[table["~{table_name}_id"].isin("~{sep='*' sample_names}".split("*"))]