sql: "no database specified" in pgwire session with database properly set

[knz]

Assuming #4537 and #4548 , investigating #4036, next error:

• JDBC sends CREATE TABLE test, this succeeds;
• next command is INSERT INTO test VALUES ..., this fails with error: no database specified: test.

Network trace:

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Parse
    Length: 13
    Statement: 
    Query: BEGIN
    Parameters: 0
PostgreSQL
    Type: Bind
    Length: 12
    Portal: 
    Statement: 
    Parameter formats: 0
    Parameter values: 0
    Result formats: 0
PostgreSQL
    Type: Execute
    Length: 9
    Portal: 
    Returns: all rows
PostgreSQL
    Type: Parse
    Length: 76
    Statement: 
    Query: INSERT INTO test ( name, val ) VALUES ( $1, $2 ) RETURNING *
    Parameters: 2
        Type OID: 1043
        Type OID: 20
PostgreSQL
    Type: Bind
    Length: 33
    Portal: 
    Statement: 
    Parameter formats: 2
        Format: Text (0)
        Format: Binary (1)
    Parameter values: 2
        Column length: 1
        Data: 61
        Column length: 8
        Data: 0000000000000000
    Result formats: 0
PostgreSQL
    Type: Describe
    Length: 6
    Portal: 
PostgreSQL
    Type: Execute
    Length: 9
    Portal: 
    Returns: all rows
PostgreSQL
    Type: Sync
    Length: 4

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Parse completion
    Length: 4

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Bind completion
    Length: 4

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Command completion
    Length: 10
    Tag: BEGIN

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Error
    Length: 48
    Severity: ERROR
    Code: XX000
    Message: no database specified: test

Internet Protocol Version 4, Src: 127.0.0.1, Dst: 127.0.0.1
PostgreSQL
    Type: Ready for query
    Length: 5
    Status: In a transaction (84)

[knz]

Waiting for #4578 to propose a standalone test (will extract from #4557).

[knz]

While looking at #4036