Add missed condition to encoding

Author: denil-ct

Sources/_OpenAPIGeneratorCore/Extensions/String.swift

@@ -82,7 +82,8 @@ fileprivate extension String {
                 outScalar = scalar
             } else {
                 var hexString = String(scalar.value, radix: 16, uppercase: true)
-                if let firstChar = hexString.unicodeScalars.first,
+                if index == 0,
+                   let firstChar = hexString.unicodeScalars.first,
                    !firstCharSet.contains(firstChar) {
                     hexString = "_\(hexString)"
                 }