Fixed inefficient cache usage.

[bart-degreed]

By turning constant expressions (whose value comes from query string) into parameters, the parsed query can be reused for varying query string parameters.

Fixes #450.

[bart-degreed]

Additional locations where Expression.Constant is used:

• 'in' filter

  JsonApiDotNetCore/src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

  Line 215 in 668f071

  var contains = Expression.Call(method, new Expression[] { Expression.Constant(obj), member });

  ,

  JsonApiDotNetCore/src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

  Line 223 in 668f071

  var notContains = Expression.Not(Expression.Call(method, new Expression[] { Expression.Constant(obj), member }));

  These operate on IList, which is translated to a SQL 'in' clause by EF Core. Although these seems to be a workaround at https://stackoverflow.com/questions/52055583/entity-framework-select-in-not-using-parameters, I feel this is better left to the EF Core team to optimize.

• 'isnull'/'isnotnull' filter

  JsonApiDotNetCore/src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

  Line 284 in 668f071

  right = Expression.Constant(null);

  Changing this makes some tests fail that try to pass null for a non-nullable ID. Having one additional cache entry for null should not be a big problem.

• ?

  JsonApiDotNetCore/src/JsonApiDotNetCore/Extensions/IQueryableExtensions.cs

  Line 389 in 668f071

  Expression.Equal(srcBody, Expression.Constant(null)),

  I found no way to get that code executed. Maybe dead code?