sql: fix resulting type for concatenation operator

Before this patch, resulting type of concatenation operator always was
TEXT (i.e. type of memory cell containing result - MEM_Str). Lets fix it
and return type depending on type of concatenation arguments: if both
arguments are TEXT, then resulting type is TEXT; BLOB otherwise. Note
that other options of combining types of arguments are illegal.

Closes #3544

src/box/sql/vdbe.c

@@ -1587,7 +1587,10 @@ case OP_Concat: {           /* same as TK_CONCAT, in1, in2, out3 */
 	if (sqlVdbeMemGrow(pOut, (int)nByte+2, pOut==pIn2)) {
 		goto no_mem;
 	}
-	MemSetTypeFlag(pOut, MEM_Str);
+	if (pIn1->flags & MEM_Str)
+		MemSetTypeFlag(pOut, MEM_Str);
+	else
+		MemSetTypeFlag(pOut, MEM_Blob);
 	if (pOut!=pIn2) {
 		memcpy(pOut->z, pIn2->z, pIn2->n);
 	}

test/sql/types.result

@@ -163,3 +163,13 @@ box.sql.execute("SELECT CAST('abc' AS BLOB) || 'x';")
 ---
 - error: 'Inconsistent types: expected BLOB got TEXT'
 ...
+-- Result of BLOBs concatenation must be BLOB.
+--
+box.sql.execute("SELECT TYPEOF(CAST('abc' AS BLOB) || CAST('cda' AS BLOB))")
+---
+- - ['blob']
+...
+box.sql.execute("VALUES (TYPEOF(randomblob(5) || zeroblob(5)));")
+---
+- - ['blob']
+...

test/sql/types.test.lua

@@ -52,3 +52,7 @@ box.sql.execute("SELECt 'a' || 'b' || 1;")
 --
 box.sql.execute("SELECT 'abc' || CAST('x' AS BLOB);")
 box.sql.execute("SELECT CAST('abc' AS BLOB) || 'x';")
+-- Result of BLOBs concatenation must be BLOB.
+--
+box.sql.execute("SELECT TYPEOF(CAST('abc' AS BLOB) || CAST('cda' AS BLOB))")
+box.sql.execute("VALUES (TYPEOF(randomblob(5) || zeroblob(5)));")