Using Groovy to set a HTTP header value

I ran into the same problem. The solution is simple: There is both StringToStringMap and StringToStringsMap in com.eviware.soapui.support.types - the method expects the plural variant.

Here's an adaption of M McDonald's script (combined with code from https://thewonggei.wordpress.com/2010/08/05/configure-http-basic-auth-once-for-soapui-test-suties/) to write TestSuite properties into the headers of all called TestRequests:

import com.eviware.soapui.impl.wsdl.teststeps.*
import com.eviware.soapui.support.types.StringToStringsMap 

for( testCase in testSuite.getTestCaseList() ) {
 for( testStep in testCase.getTestStepList() ) {
  if( testStep instanceof WsdlTestRequestStep ) {
   def headers = new StringToStringsMap()
   headers.put(testSuite.getPropertyValue("ExampleHeaderName"),testSuite.getPropertyValue("ExampleHeaderValue"))

   log.info("Setting HTTP headers ${headers} in test case ${testCase.getLabel()}")
   testStep.getTestRequest().setRequestHeaders(headers)
  }
 }
}

Just set the properties ExampleHeaderName and ExampleHeaderValue for your TestSuite and use the script as "Setup Script" of the TestSuite.