Removed second FilterStringKeyValue method and improved first

Second method was non-functional due to access modifier inconsistency, added IsNullOrEmpty and MaxLobbyKeyLength check to first method, added MaxLobbyKeyLength (and backing field) to SteamMAtchmaking, taken from https://partner.steamgames.com/doc/api/ISteamMatchmaking#constants

Facepunch.Steamworks/SteamMatchmaking.cs

@@ -12,6 +12,14 @@ namespace Steamworks
 	/// </summary>
 	public static class SteamMatchmaking
 	{
+		internal const int k_nMaxLobbyKeyLength = 255;
+
+		/// <summary>
+		/// Maximum number of characters a lobby metadata key can be
+		/// </summary>
+		public static int MaxLobbyKeyLength { get { return k_nMaxLobbyKeyLength; } }
+
+
 		static ISteamMatchmaking _internal;
 
 		internal static ISteamMatchmaking Internal

Facepunch.Steamworks/Structs/LobbyQuery.cs

@@ -51,21 +51,14 @@ namespace Steamworks.Data
 		/// </summary>
 		public LobbyQuery FilterStringKeyValue( string nKey, string nValue )
 		{
-			stringFilter = new MatchMakingKeyValuePair_t
+			if ( !string.IsNullOrEmpty(nKey) && nKey.Length <= SteamMatchmaking.MaxLobbyKeyLength )
 			{
-				Key = nKey,
-				Value = nValue
-			};
-
-			return this;
-		}
-
-		/// <summary>
-		/// Filter by specified key/value pair; MatchMakingKeyValuePair_t parameter
-		/// </summary>
-		public LobbyQuery FilterStringKeyValue( MatchMakingKeyValuePair_t kv )
-		{
-			stringFilter = kv;
+				stringFilter = new MatchMakingKeyValuePair_t
+				{
+					Key = nKey,
+					Value = nValue
+				};
+			}
 
 			return this;
 		}
@@ -99,6 +92,7 @@ namespace Steamworks.Data
 
 		#endregion
 
+
 		void ApplyFilters()
 		{
 			if ( distance.HasValue )