make character case for keys in parameters map irrelevant, fixing #143

More specifically,
- introduce helper function to get maps with all keys set to lowercase,
- introduce lookup helper based on such maps and
- change lookups for CreateVolumeRequest()s and CreateVolume()s so that
  parameter keys are processed as lowercase irrespective of actual
  spelling.

Signed-off-by: Christopher J. Ruwe <cjr@cruwe.de>

pkg/common/helpers/helpers.go

@@ -0,0 +1,43 @@
+/*
+Copyright 2020 The OpenEBS Authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package helpers
+
+import (
+	"strings"
+)
+
+// Coerce the map's keys to lower case, which only works when unicode char is in
+// ASCII subset. May overwrite key-value pairs on different permutations of key
+// case as in Key and key. DON'T force values to the lower case unconditionally,
+// because values for keys such as mountpoint or keylocation are case-sensitive.
+// Note that although keys such as 'comPREssion' are accepted and processed,
+// even if they are technically invalid, updates to rectify such typing will be
+// prohibited as a forbidden update.
+func GetCaseInsensitiveMap(dict *map[string]string) map[string]string {
+	insensitiveDict := map[string]string{}
+
+	for k, v := range *dict {
+		insensitiveDict[strings.ToLower(k)] = v
+	}
+	return insensitiveDict
+}
+
+// special case of GetCaseInsensitiveMap looking up one key-value pair only
+func GetInsensitiveParameter(dict *map[string]string, key string) string {
+	insensitiveDict := GetCaseInsensitiveMap(dict)
+	return insensitiveDict[strings.ToLower(key)]
+}