Added -Exact flag to Get-JiraUser

If JIRA has usernames such as "jsmith", "jsmithy", "jsmith1", "jsmithson" and you are trying to assign the ticket to "jsmith" it will fail because the assignee field needs a string but this function is returning an array of strings.
By adding the Exact flag to the Get-JiraUser function this issue is resolved as only one user object is returned.

JiraPS/Public/Invoke-JiraIssueTransition.ps1

@@ -109,7 +109,7 @@ function Invoke-JiraIssueTransition {
                 $validAssignee = $true
             }
             else {
-                if ($assigneeObj = Get-JiraUser -InputObject $Assignee -Credential $Credential) {
+                if ($assigneeObj = Get-JiraUser -InputObject $Assignee -Credential $Credential -Exact) {
                     Write-Debug "[$($MyInvocation.MyCommand.Name)] User found (name=[$($assigneeObj.Name)],RestUrl=[$($assigneeObj.RestUrl)])"
                     $assigneeString = $assigneeObj.Name
                     $validAssignee = $true